i've probably written on this before but this article in the ny times inspired tonite's blog: http://well.blogs.nytimes.com/2010/10/13/phys-ed-do-marathons-wreck-your-knees/?src=un&feedurl=http%3A%2F%2Fjson8.nytimes.com%2Fpages%2Fmagazine%2Findex.jsonp
i alert you to the concluding line: ‘There’s no strong evidence,’’ he said, that, if your knees are healthy to start with, ‘‘running a marathon will hurt them.’’
i think it's fair to say, as i've long held, that there are no wrong exercises for everyone but there are wrong exercisers for some exercisers. in other words, using myself as an example, having done tae kwon do nearly every day of the year for about 15 years, that my knee (only my pivot leg) is shot because i must have either trained wrong or had a propensity toward arthritis. (i also have a new hip on the same side and the knee will someday need replacing.) my friend, same age, same duration of youthful training, is still teaching, running, and sparring (gently, because we are, after all, old men) and is free of knee problems. (tho he's had both his hips re-surfaced. it's a tough sport...) my point is, some suffer consequences, others don't. same for knees and running.
the studies reported in this article are noteworthy for their longevity. rather than a cross sectional study of runners and non-runners, or runners and former runners, some of these in this piece followed runners over a period of time. what they generally found was that running did not cause arthritis, but it did alter the 'matrix' of the cartilage. however, that may be one of the many ways our body adapts to the stresses of long distance long-term running. obviously, some runners had to give it up because their knees did not adapt well. others are still running. thus, i think it's fair to conclude that, much as that last line says, running does not cause arthritis unless your knees are prone to arthritis.
now, many things can make you prone: genetics, previous injury, untreated training or footwear conditions (running on the same side of the street, or in the wrong shoes - old or inappropriate for your foot), or any number of nutritional or extra-activity variables that can't be determined. (for example, is my knee shot because i also tried to be a distance athlete? or because i worked out on tile floors 3 hrs/day? or because i needed orthotics for running but trained barefoot? who knows????)
so, in conclusion, let me state clearly my position and highlight that of the news piece: exercise however you wish. pay attention to your body. treat it like a living machine, providing it the elements of success under duress - fluids, nutrients, and rest. and maybe just maybe you will not be one of those who suffers cartilage damage from your activity of choice.
but maybe you will regardless.